Pesticides kill competing insects or weeds that eat our food, but we do not want to get sick or be killed by them. Therefore, finding out how much pesticides are in our food is key to food safety.

As food involves so many complex substances, equipment in labs certainly could not analyze food in their raw forms. Therefore, we need to separate pesticides from sample for measurements, which is the sample preparation process. The traditional sample preparation methods require long labour hours and large sample sizes. Here, the authors proposed a Quick, Easy, Cheap, Effective, Rugged, and Safe method called “QuEChERS”, which is nowadays a common practice of sample preparation for fruits and vegetables.

Here is the author’s proposal in a nutshell (Fig. 1): 10 g of homogenized sample are put into organic solvents and salts which helps dig out pesticides from food chunks to the solvents. This is the extraction process. Then, the extract is cleaned to reduce the food components that messed up with our target signals in instrumental analysis. This second part is the cleanup.

Figure 1 Flow chart of QuEChERS procedures

Extraction

The aim of this process is to extract pesticide compounds from food to organic solvents. Most of our food contains much water. To separate the pesticides from food and water within, we need to customise our methods. This include the smart choice of two key helpers in the process, solvents and salts. Solvents are used for extraction of pesticides from food samples, and salts are used to saturate the water layer for better extraction. There are three popular candidates for solvents, ethyl acetate, acetone and acetonitrile. Ethyl acetate is only partially immiscible with water, which does not favor the extraction of many water-soluble pesticide compounds. Therefore, it requires additional helper such as anhydrous sodium sulphate salt or methanol. Acetone, on the other hand, mixes too well with water that it requires additional solvents to separate it from water layer. Acetonitrile is the best remove suit candidate. It mixes well with water. Yet, upon addition of salts, such as anhydrous magnesium sulphate and sodium chloride, acetonitrile can be well-separated from water. Its low volatility also minimizes the loss during transfer of sample extract.

Cleanup

Other food components such as sugars, fats, pigments often complicate the instrument signals. In order to have reliable measurements, these need to be removed through a cleanup step. The conventional approach is called solid phase extraction (SPE), which involves the use of small columns packed with absorbing materials (sorbent). It first washes away the unwanted components by sticking the pesticides on sorbent. Then, with the help of another solvents to force the pesticides out. This, requires the use of much solvents. Here, the authors propose an easy alternative called the dispersive Solid Phase Extraction (dSPE). Instead of adding sample extract to packed sorbent materials, they were added to a powder form of “cleanup” salts. This can tremendously reduce the amount of solvents required and increase efficiency. The mixture is then vortexed or shaken to mix them well (Fig. 2).

Figure 2 The difference between SPE and dSPE procedure. On the left, sample extract is added to the sorbent materials in a column. The pesticides are first absorbed to the sorbents while washing away the non-targeted substances. The pesticides are then elute out from the sorbents. On the right, sample extract is added to dSPE salts. They are shaken or vortexed to let the unwanted substances to be absorbed by the sorbents, thereby cleaning up the extract.

Several dSPE salts have been investigated. Graphitized Carbon Black (GCB) attracts planar molecules strongly which benefit the removal of pigments and sterols. However, this could filter out our target pesticides if they happen to be planar as well. Primary Secondary Amines (PSA) has many amino acid groups which fascilitate the removal of compounds that can form hydrogen bonds, such as compounds with hydroxy or carboxy groups (fatty acids and carbohydrates).

Conclusion

This extraction-cleanup method has become a popular sample preparation routine for pesticides examination all around the world. The simple and easy sample preparation steps have tremendously increased the efficiency of testing. However, it is just a general outline for sample preparation. There are tricky pesticides that are sensitive to the types of solvents and salt, as well as external conditions such as temperature and pH. This is exactly where professional chemists come into place. Their insights and experience will always find a way to reveal the harmful chemicals in our food.

I have always enjoyed the thrill of learning something new. It is this pursue that propels me to move from one chapter to another: from undergraduate physics, to PhD in inorganic and computational chemistry, and an R&D chemist in analytical chemistry. I am now driven by my curiosity to start my new chapter in science communication, and eagerly to share my wonders of the world with you!